1. Field of the Invention
The invention concerns method and apparatus for longitudinal orientation of thermoplastic film material, especially with a view to the manufacture of crosslaminates of films which have been uniaxially stretched at a relatively low temperature. It is known that the best all-round strength properties in a crosslaminate are obtained by the following steps of orientation: first a strong almost uniaxial melt-orientation during the drawdown from the extrusion die, or still better an almost uniaxial orientation while the polymer material is semimolten, and then further orientation at a rather low temperature. “All-round strength properties” here refers to a combination of tensile strength, yield point, tear propagation resistance and puncture resistance. It is difficult to give a satisfactory explanation why this combination of orientation steps is preferable, but it can briefly be said that when the orientation is carried out in these steps, the molecular chains will exhibit a wide spectrum of different degrees of orientation, and those of relatively low orientation will help the film to re-orient instead of splitting, when it is subjected to tearing or puncturing forces.
However, the stretching at low temperature causes significant problems, e.g. in films which may consist of high density polyethylene (HDPE) or isotactic or syndiotactic polypropylene (PP). One side of this problem is that, when a film is longitudinally stretched, it has a high tendency to contract in the transverse direction, at the same time as its thickness is reduced. This tendency is highest when the temperature is low, e.g. between 10-40° C. which is optimum stretching temperature range for HDPE and PP, as far as the achieved properties are concerned. The other side of the problem is that, at these low temperatures the material tends to “neck in”, instead of gradually developing the orientation within a reasonably long zone. This means that the stretching must take place between closely spaced stretching rollers or stretching bars, and unless special precautions are taken this will prevent the film from undergoing the needed contraction in the transverse direction.
2. Description of the Related Art
In the inventor's U.S. Pat. No. 3,233,029, which was published about 40 years ago, a proposal is made for solution of this problem, namely, to “anticipate” a substantial part of the transverse contraction to which the film tends by longitudinal pleating prior to a stretching within one or more short stretching zones, as this is more exactly expressed as follows: the stretching taking place in one or more short zones between and/or on two or more stretching rollers or bars, in which process the width measured in a straight line from edge to edge is reduced prior to the stretching, this reduction being in form of a regular pattern of longitudinally extending pleats, whereby the reduction of width and the length of the stretching zones are adapted to allow the film a full straightening out of the pleats by the inherent tendency in the polymer material to contract transversely while being stretched longitudinally.
In that patent, the pleating mechanism described consists of two sets of discs which are mounted spaced apart upon the shafts, one over and one under the film to become pleated, so that discs in one set mesh between the discs in the other set. Thereby the film is forced to form folds or convolutions. It is further disclosed that the film preferably shall pass over a crown-shaped roller adapted to make the stress upon the borders equal to that in the middle of the film. Crown-shaping means that the roller has highest diameter in its middle, the diameter gradually decreasing towards its ends. Finally it is disclosed that the film preferably is cooled in the stretching zone, which may conveniently be by covering a stretching bar with felt and keeping this felt wet. The water also, by its lubricating action, helps to allow the film the transverse contraction which eliminates the pleats. No pleats remain in the final product.
The inventor managed to make this old invention work with flexibilized HDPE and PP, but only in relatively narrow widths, insufficient for an industrial production e.g. of cross-laminated industrial bags or cross-laminated coversheet. When trying to apply the invention to stiffer film, such as film made from plain HDPE or PP, or when trying it on film of greater width, e.g. 1 m wide, the transverse forces applied by the film always caused a transverse stretching of the film in the form of thin, longitudinally extending lines. It appears that the principle to apply longitudinal pleating thereby allowing a film transverse contraction during longitudinal stretching, has up to now only been carried out industrially under conditions which also produce transverse stretching and attenuation along narrow longitudinal lines.